This invention relates to power supply systems in which a plurality of DC power supplies are arranged to supply a common load, at least one of the power supplies being redundant during normal operation such that the system is capable of continuing to supply the load in the event of failure of one of the power supplies.
It is customary for each power supply to be provided with a respective isolation device such as a diode to prevent any unwanted backflow of current under any circumstances where the power supply to which it is connected is inoperative, whether intentionally or due to malfunction.
A latent fault may exist if an isolation device fails, for example by developing a short circuit fault, there being apparently no overall change in the operational status of the system while its power supply continues to be operative. Since, however, the integrity of the isolation device is relied upon in the event of power supply failure, the existence of a latent fault can have serious repercussions. Such a fault may result in undesirable reverse current through a defective power supply if the protection provided by the isolation device is removed due to short circuit failure with the potential for causing a catastrophic failure of other power supplies of the system or causing damage to the load, as for example in the case where the system powers a computer system.
U.S. Pat. No. 5,548,463 discloses detection of a latent short circuit fault in a power mixing device by momentarily turning off power on a first rail and monitoring a test node on the first rail between the power supply and the power mixing device to detect a momentary return to ground at the test node. This sequence is then repeated for corresponding elements of a second rail. For many systems, however, such interruption of a power supply with its associated transients may be unacceptable.